In a typical land mobile radio system, such as a mobile cellular telephone system or personal communications network (PCN), a plurality of cells are defined which make up the system. Each cell is a geographically defined area wherein communications are handled by a land mobile radio base site (cell site) for mobile units operating within the boundaries of the cell. Although these cells are often represented as hexagons in cell design schemes, in reality, due to terrain and the presence of buildings and other structures, the actual boundary of a cell may have an irregular shape.
As is well known in the art, cell layouts are typically characterized by a frequency reuse pattern where a number of different frequency sets are defined. Each cell uses a particular frequency set, and the cell layout is designed to provide the maximum separation between cells using the same frequency set so as to minimize interference.
As a mobile unit travels along a path that passes from one cell to another, a handoff occurs from one cell site to another. The handoff action is controlled by a mobile telephone switching office (MTSO). A handoff command is typically generated when the signal received at a cell site from a mobile unit falls below a preselected signal strength thus indicating that the mobile unit is at a cell boundary. As the mobile unit passes from one cell to another, the MTSO instructs the cell which the mobile unit is entering to begin transmitting at a frequency which is different from the frequency which was being transmitted by the cell which the mobile unit is leaving. This procedure is repeated as the mobile unit travels from cell to cell.
A mobile unit typically contains a control unit, a transceiver, and an antenna system. Each cell site typically includes control means, a plurality of radio channel units (radios), a power plant, a data terminal, and antennas. The MTSO provides coordination for all of the cell sites and contains suitable processing and switching means. The communication links between the MTSO and the various cell sites are typically microwave, T carriers, or optical fiber, and carry both voice and control data between the cell sites and the MTSO. The MTSO that controls the cell sites also normally provides connectivity to nationwide telephone networks.
It is generally required, depending on the location of the cell site within a cell, that the cell site antennas provide coverage for communications over 360.degree. of azimuth in order to effectively cover the corresponding geographic area. Typically, the 360.degree. of azimuth is divided into a number of smaller sectors, and antennas having a directional beam are provided to service each sector, each antenna having a beamwidth corresponding to the azimuth of the sector. For example, if three sectors are defined, each sector is provided with a pair of antennas each having a beamwidth of 120.degree..
In the above described land mobile radio base site, each radio channel unit is a transceiver, having a transmit section and a receive section, able to transmit and receive RF signals, respectively. The transmit section of each radio channel unit is directly connected to one antenna within one of the sectors for the transmission of RF signals, and the receive section is directly connected to both antennas in the sector for the receipt of RF signals. All of the radio channel units connected to a particular pair of antennas within a sector are grouped together. For example, in a land mobile radio base site having 60 radio channel units and 3 pairs of antennas in three 120.degree. sectors, the radio channel units are divided into three groups of 20, with all of the radio channel units within a group being connected to the same antenna pair. The group of radio channel units assigned to a pair of antenna is call a "trunk group".
When a mobile unit transmits a call request, the request is received by a cell site, and the cell site selects the best directive antenna for the voice channel to use. At the same time, the cell site sends a request to the MTSO via a high-speed data-link. The MTSO selects an appropriate voice channel for the call, and directs the cell site to act on the selection through the best directive antenna and one of the radio channel units in the antenna's trunk group. The MTSO also acts on the call request by making the appropriate connection with a telephone network.
When the mobile unit is traveling from one cell to another, a handoff occurs as described above. When the mobile unit is traveling within a cell, all of the antennas within the cell receive signal levels, but not necessarily the voice signals from the mobile unit. Only the antenna within the sector where the call was established (where the received signal level was the strongest) will transmit and receive signals to the mobile unit. When the mobile unit moves within the cell so that another sector receives the strongest signal, the system turns off the radio channel unit in the weaker sector and turns on a radio channel unit in the stronger sector such that the call is serviced by a new radio channel unit and antenna pair in a different sector within the cell. Typically, this change within a cell is handled solely by the base site control means, and the MTSO is not involved.
Usage capacity for a cell site is defined as the percentage of total available air time which can be statistically used over a given period of time. In the above described base site having 60 radio channel units, the total available air time during a one hour period is 60 units.times.60 minutes=3600 minutes, and the usage capacity is the percentage of that 3600 minutes which, statistically, can be used in a one hour period. The usage capacity will be less than 100% based on numerous factors, but most importantly based on the probability that all of the radio channel units in a given trunk group will be busy. A field of mathematics called "queuing theory" teaches that the usage capacity of a trunk group increases non-linearly with the number of radios in the trunk group. See, e.g., Bert et al., Data Networks 2nd Edition, pages 174-179. Therefore, it is desirable to avoid a large number of small trunk groups because they are relatively inefficient as compared to a smaller number of large trunk groups.
It is well known in the art that the transmission and reception of RF signals from a base site may be improved by using directional antenna having a narrow beamwidth, e.g., 30.degree. as opposed to 120.degree.. This improvement is due to a number of factors. A first important factor is a reduction in interference. Since energy is primarily transmitted in the known direction, i.e., the narrow beamwidth of the antenna, the base site produces much less RF interference for mobile units outside of the antenna's beam. Additionally, since the base site receives energy primarily from the direction of the mobile unit, the reception of undesired signals is greatly reduced. Therefore, increased directivety of an antenna allows interference reduction in both directions.
In addition to reduced interference, an improvement in geographic coverage is attained. Geographic coverage relates to the geographic area surrounding the base site wherein the magnitude of an RF signal transmitted from the base site and received by a mobile unit is strong enough to overcome the electrical noise present in the mobile unit's receiver by a predetermined amount. The amount is typically 17 dB, e.g., the signal received at the mobile unit from the base site must exceed the noise floor of the receiver by 17 dB. The directivety and gain of an antenna are related to its vertical and azimuthal beamwidths. For an omni-directional antenna, e.g., an antenna having a 360.degree. beamwidth, the antenna gain can only be increased by narrowing its vertical beamwidth. Such omni-directional antennas have a typical maximum gain of around 10 to 12 dB relative to an isotropic (spherical) radiator. With sector antennas having beamwidths of between 90.degree. and 120.degree., the achievable gains are around 15 dB with respect to an isotropic radiator. Using narrow beamwidth antenna, e.g., 15.degree. to 30.degree. beamwidths, gains of around 26 dB relative to an isotropic radiator are achievable. From the above it can be seen that narrow beamwidth antennas provide the significant advantage of improved geographical coverage with respect to antenna having a wider beamwidth.
In prior art systems, the advantages of using narrow beamwidth antennas must be balanced against the loss in usage capacity of using a large number of small trunk groups (associated with narrow beamwidth antennas). The present invention provides a solution to this problem by providing for the interconnection of any radio channel unit in a large trunk group with any beam of a plurality of antennas.